With this feature one can specify keys that behave differently, based on
the amount of times they have been tapped, and when interrupted, they
get handled before the interrupter.

To make it clear how this is different from `ACTION_FUNCTION_TAP`, lets
explore a certain setup! We want one key to send `Space` on single tap,
but `Enter` on double-tap.

With `ACTION_FUNCTION_TAP`, it is quite a rain-dance to set this up, and
has the problem that when the sequence is interrupted, the interrupting
key will be send first. Thus, `SPC a` will result in `a SPC` being sent,
if they are typed within `TAPPING_TERM`. With the tap dance feature,
that'll come out as `SPC a`, correctly.

The implementation hooks into two parts of the system, to achieve this:
into `process_record_quantum()`, and the matrix scan. We need the latter
to be able to time out a tap sequence even when a key is not being
pressed, so `SPC` alone will time out and register after `TAPPING_TERM`
time.

But lets start with how to use it, first!

First, you will need `TAP_DANCE_ENABLE=yes` in your `Makefile`, because
the feature is disabled by default. This adds a little less than 1k to
the firmware size. Next, you will want to define some tap-dance keys,
which is easiest to do with the `TD()` macro, that - similar to `F()`,
takes a number, which will later be used as an index into the
`tap_dance_actions` array.

This array specifies what actions shall be taken when a tap-dance key is
in action. Currently, there are two possible options:

* `ACTION_TAP_DANCE_DOUBLE(kc1, kc2)`: Sends the `kc1` keycode when
  tapped once, `kc2` otherwise.
* `ACTION_TAP_DANCE_FN(fn)`: Calls the specified function - defined in
  the user keymap - with the current state of the tap-dance action.

The first option is enough for a lot of cases, that just want dual
roles. For example, `ACTION_TAP_DANCE(KC_SPC, KC_ENT)` will result in
`Space` being sent on single-tap, `Enter` otherwise.

And that's the bulk of it!

Do note, however, that this implementation does have some consequences:
keys do not register until either they reach the tapping ceiling, or
they time out. This means that if you hold the key, nothing happens, no
repeat, no nothing. It is possible to detect held state, and register an
action then too, but that's not implemented yet. Keys also unregister
immediately after being registered, so you can't even hold the second
tap. This is intentional, to be consistent.

And now, on to the explanation of how it works!

The main entry point is `process_tap_dance()`, called from
`process_record_quantum()`, which is run for every keypress, and our
handler gets to run early. This function checks whether the key pressed
is a tap-dance key. If it is not, and a tap-dance was in action, we
handle that first, and enqueue the newly pressed key. If it is a
tap-dance key, then we check if it is the same as the already active
one (if there's one active, that is). If it is not, we fire off the old
one first, then register the new one. If it was the same, we increment
the counter and the timer.

This means that you have `TAPPING_TERM` time to tap the key again, you
do not have to input all the taps within that timeframe. This allows for
longer tap counts, with minimal impact on responsiveness.

Our next stop is `matrix_scan_tap_dance()`. This handles the timeout of
tap-dance keys.

For the sake of flexibility, tap-dance actions can be either a pair of
keycodes, or a user function. The latter allows one to handle higher tap
counts, or do extra things, like blink the LEDs, fiddle with the
backlighting, and so on. This is accomplished by using an union, and
some clever macros.

In the end, lets see a full example!

```c
enum {
 CT_SE = 0,
 CT_CLN,
 CT_EGG
};

/* Have the above three on the keymap, TD(CT_SE), etc... */

void dance_cln (qk_tap_dance_state_t *state) {
  if (state->count == 1) {
    register_code (KC_RSFT);
    register_code (KC_SCLN);
    unregister_code (KC_SCLN);
    unregister_code (KC_RSFT);
  } else {
    register_code (KC_SCLN);
    unregister_code (KC_SCLN);
    reset_tap_dance (state);
  }
}

void dance_egg (qk_tap_dance_state_t *state) {
  if (state->count >= 100) {
    SEND_STRING ("Safety dance!");
    reset_tap_dance (state);
  }
}

const qk_tap_dance_action_t tap_dance_actions[] = {
  [CT_SE]  = ACTION_TAP_DANCE_DOUBLE (KC_SPC, KC_ENT)
 ,[CT_CLN] = ACTION_TAP_DANCE_FN (dance_cln)
 ,[CT_EGG] = ACTION_TAP_DANCE_FN (dance_egg)
};
```

This addresses #426.

Signed-off-by: Gergely Nagy <algernon@madhouse-project.org>

Signed-off-by: Gergely Nagy <algernon@madhouse-project.org>

Process the tap dance stuff after midi and audio, because those don't
process keycodes, but row/col positions.

Signed-off-by: Gergely Nagy <algernon@madhouse-project.org>

To be consistent with how the rest of the quantum features are
implemented, use ifdefs instead of dummy functions.

Signed-off-by: Gergely Nagy <algernon@madhouse-project.org>